This invention relates generally to a method of controlling open-angle glaucoma by surgery and more specifically to the use of a laser in surgery to reduce the thickness of the trabecular meshwork and tissue around Schlemm's Canal to increase filtration of the aqueous humor and thereby controlling the open-angle glaucoma.
Primary open-angle glaucoma is a disease of unknown etiology known also as simple glaucoma, chronic glaucoma, glaucoma simplex, compensated glaucoma, and open angle glaucoma. The disease is characterized by the increase in intraocular pressure which results in atrophy of the optic nerve, visual field disturbances, and eventual blindness. In primary open-angle glaucoma the anterior-chamber angle appears normal to direct observation, and the aqueous humor has free access to the trabecular meshwork. Secondary open-angle glaucoma can occur because of fibrovascular proliferation and the trabecular meshwork is abnormal due to other eye diseases. Obstruction of the trabecular meshwork prevents the filtration of aqueous humor with a resulting increase in intraocular pressure.
Open angle glaucoma can be treated medically or surgically. The preferred treatment is medical and is directed toward increase in The outflow of aqueous humor from the anterior chamber of the eyeball or by decreasing the secretion of aqueous humor, or both.
Primary open-angle glaucoma is treated with drugs such a timolol, an adrenergic receptor antagonist; pilocarpine, a cholinergic stimulating drug; echothiophate iodide, a cholinesterace antagonist; epinephrine, an alpha and beta agonist; and acetazolamide, a carbonic anhydrase inhibitor.
There are problems inherent with medical treatment. Miotic drugs like pilocarpine may aggravate the visual loss caused by incipient cataract or may induce painful ciliary muscle spasm. Epinephrine may be irritating to the eye. Echothiophate iodide has a myriad of adverse effects, drug interactions, and contraindications, as does acetazolamide. Timolol is contraindicated in patients with asthma and other pulmonary diseases.
If the disease cannot be controlled by drugs and there is progress in the associated visual field defects and severity of optic nerve atrophy, surgery is indicated.
The main surgical procedure in open-angle glaucoma in which the trabecular meshwork is visible is laser trabeculoplasty, commonly using an argon laser. The eye is anesthetized and the trabecular meshwork is visualized through a gonioprism. The laser energy is applied ab externo, to photocoagulate the trabecular meshwork, often 50 to 100 spots, spaced evenly over the entire circumference of the anterior trabecular meshwork. The laser can reduce the circumference of the trabecular ring by heat induced shrinkage of the collagen of the sheet of trabecular tissue or by scar tissue contraction at the burn sites, forcing the ring to move toward the center of the anterior chamber, elevating the sheets, and pulling open the intertrabecular spaces. Flow is increased through the trabecular spaces. The main complication in this type of surgery is a transient increase in intraocular pressure that may require medication to control. Control is usually achieved in about 85% of all patients, but most (75%) continue to require medicines. However, control can be lost with the passing of time and additional laser trabeculoplasty may not be effective.
In eyes where laser trabeculoplasty cannot be performed or where it fails to control pressure, a filtering operation is indicated. All previous filtering operations were based on the theory of creating a fistula between the anterior chamber and the sub-conjuctival space through which aqueous humor can flow. Generally, the surgery was performed by scalpel. Trabeculectomy is the operation of choice. An operating microscope is used and a scleral flap is fashioned to expose the trabecular meshwork. A portion of the meshwork is exised and the scleral flap is replaced. A filtering bleb often develops after surgery. This surgery is performed in an operating surgery suite, and complications can include excessively low intraocular pressure, flat anterior chamber, endophthalmitis, cataract, sympathetic ophthalmic and bullous keratopathy. Furthermore, a mechanical disection has limited success because of the difficulty in judging the depth of dissection of the trabecular meshwork and the surgeon can inadvertently enter the anterior chamber with his surgical tool.